FAQ

Q: What’s the difference between April CU and SP2?
A: Office Sustained Engineering team explained the difference on their blog: FAQ concerning Service Pack 2 and the Cumulative Updates for April 2009 for the 2007 Microsoft® Office System and Microsoft Office Servers. To be simple: April CU contains fixes that are not part of SP2. But it includes only a subset of all the files. So if you want to get everything in your farm up-to-date, you need to apply SP2 first, then apply April CU.

Q: Do you have a timeline on the updates? I’m confused.
A: Here’s an example timeline to help you understand the update schedule during 2007~2009. Cumulative Updates are released every two months. Start from April 2009, cumulative updates cannot be applied directly on RTM version of SharePoint installations. SP1 is the minimum requirement, while SP2 is recommended.

Reference http://blogs.msdn.com/sharepoint/default.aspx

Ivan